In the past, graphite reinforced composites utilized for aircraft structural applications such as stabilizers, flaps and the like have been made with a thermosetting resin matrix. Each part is hand laid up, bagged and cured in an autoclave. One difficulty with such thermosetting laminates, however, is that once the laminate has been formed into a desired shape, it cannot be reprocessed if defects are present. Consequently, when a thermosetting composite is employed, the entire part must be scrapped or subjected to expensive repair procedures if it is defective. Another disadvantage of thermosetting resins is that in order to form the laminate, a reinforcement must be preimpregnated with the thermosetting resin, laminated and then cured with heat and pressure. The amount of heat and heating time employed and the pressure during cure are extremely critical since care must be taken to insure that the curing reaction proceeds at the proper rate. Furthermore, the thermosetting preimpregnated reinforcements have a finite shelf life, usually 6 to 12 months.
Such laminates can be used alone or with metal cladding. Metal-clad fiberglass laminates and the like have been used primarily in aircraft construction in attachment areas, shear panels and access areas. The advantage metal-clad laminates have over single composites is that they enable more reliable and more conventional attachment techniques such as bolting, riveting and the like. Furthermore, they improve load transfer characteristics and have better abrasion and damage resistance. Here again, however, because of the permanent set of a thermosetting resin, metal-clad laminates have been limited to flat or relatively simple contours and constructions; and it has not been possible to form thermosetting composite laminates by heat and pressure from a flat configuration.
In the past, formable metal-clad composite laminates using thermoplastic resins have been employed; however these relatively low fiber reinforcement volumes are not ordinarily utilized as structural components.